Nutrient availability alters levels of non-translationally synthesized nitrogen-rich dipeptides in Emiliania huxleyi

The coccolithophore Emiliania huxleyi synthesizes the N-rich dipeptides glutaminecysteine (gln-cys) and arginine-cysteine (arg-cys) in response to metal stress, but it is unknown how these compounds are synthesized and whether these dipeptides may also be involved in N assimilation or N storage. Accumulation of dipeptides and other reduced thiols was much greater in P-limited than N-limited batch cultures, and levels of dipeptide and cysteine were higher in an open-ocean strain compared to a coastal isolate. A comparison of dipeptide concentration to total N, protein, and amino acid content of N- and P-limited batch cultures of both strains was used to yield clues about strain-specific differences in nitrogen storage. Both gln-cys and arg-cys quickly accumulated following the addition of nitrate or ammonium to N-limited batch cultures. Accumulation of dipeptides was not suppressed by the ribosomal inhibitor cycloheximide, except at the earliest time point following ammonium addition, and, after 24 h, the inhibitor had either no effect on intracellular concentrations or instead enhanced accumulation. Results suggest that accumulation of dipeptides does not require de novo synthesis of protein and that an enzymatic pathway is responsible for synthesis. Rapid de - gradation of intracellular gln-cys and arg-cys was observed following the addition of phosphate to P-limited batch cultures. Possible synthetic pathways and ties to N metabolism are discussed. & copy Inter-Research 2011.